As a wing moves through the air, the airfoil is inclined to the
flight direction at an angle.
The angle between the chord line
and the flight direction is called the angle of
attack and has a large effect on the
drag
generated by the wing.

The magnitude of the drag generated
by an object
depends
on the
shape
of the object and how it moves
through the air. For thin airfoils, the drag is
nearly constant at small angles (+/- 5 degrees). As the angle
increases above 5 degrees, the drag quickly rises because of
increased frontal area and increased
boundary layer
thickness. As an object moves through the air, air
molecules
stick
to the surface. This creates a layer of air near the
surface called a boundary layer which, in effect, changes
the shape of the object. The flow reacts to the edge of the boundary
layer just as it would to the physical surface of the object. To make
things more confusing, the boundary layer may lift off or "separate"
from the body and create an effective shape much different from the
physical shape.
When the boundary layer separates, the wing is said to
be stalled and
both drag and
lift
become unsteady. Determining the drag is very difficult under stalled
conditions.

On the slide, the flow conditions for two airfoils are shown on
the left. The shape of the two foils is the same; the lower foil is
inclined at ten degrees to the incoming flow, while the upper foil is
inclined at twenty degrees. On the upper foil, the boundary layer has
separated and the wing is stalled. Predicting the stall point,
the angle at which the wing stalls, is very difficult mathematically.
Engineers usually rely on wind tunnel
tests to determine the stall point. But the test must be done very
carefully, matching all the important similarity
parameters. of the actual flight hardware.

The plot at the right of the figure shows how the drag varies with
angle of attack for a typical thin airfoil. At low angles, the drag is
nearly constant. Notice on this plot that at zero angle, a small
amount of drag is generated because of skin friction and the airfoil
shape. At the right of the curve, the drag changes rather abruptly
and the curve stops. In reality, you can set the airfoil at any angle
you want. However, once the wing stalls, the flow becomes highly
unsteady and the value of the drag changes rapidly with time.
Because it is so hard to measure such flow conditions, engineers
usually leave the plot blank beyond wing stall.

Since the amount of drag generated at zero angle and the location
of the stall point must usually be determined experimentally,
aerodynamicists include the effects of inclination
in the drag coefficient.
But this presents an additional problem.
There is another factor which affects the amount of drag produced
by a finite wing. The effect is called
induced drag
or drag due to lift. The flow around the wing tips of a finite wing
create an "induced" angle of attack on the wing near the tips.
As the angle increases, the lift
coefficient increases and this changes the amount of the induced
drag. To separate the effects of angle of attack on drag, and drag
due to lift, aerodynamicists often use two wing models. The wing model
to determine angle of attack effects is long and thin, and may span the
entire tunnel to produce a "two-dimensional" airfoil. Another model is
used to determine the effects of the wing tips on the drag.

You can further investigate the effect of angle of attack and the other
factors affecting drag by using the
FoilSim III Java Applet.
You can also
download
your own copy of FoilSim to play with
for free.